Forming watermarks in identification cards typically involves printing dark text or a dark image inside an opaque, white core. The process generally includes using two layers of white material, such as poly(vinyl chloride) or a poly(vinyl chloride) composite. In one example, a visual preprint is added on a first side of the first white layer, and the watermark is printed on the opposite side of the first white layer. The first white layer and the second white layer are fused together, with the watermark of the first white layer facing the second white layer, thereby embedding the watermark in the core of the identification card. When the identification card is held up to a strong light, or backlit, the watermark can be viewed through the front of the card and the back of the card.
Other visible markings on the identification card (e.g., images, text, preprinted patterns) are usually opaque and therefore can interfere with viewing the watermark. For the watermark to be visible, the dark text or image is typically selected to have a large area, such that the watermark can be distinguished from the other visible markings on the identification card. Accordingly, edge definition and detail are generally difficult to attain with watermarks formed by this technology.